1. Field of the Invention
This invention relates to ceramic wiring boards and more particularly to wiring boards wherein at least the conductor filled in through-hole parts of ceramic substrates is improved.
2. Brief Description of the Prior Art
Hitherto, ceramic wiring boards have been made by screen printing a wiring pattern with a conductor paste containing a metal powder selected from W powder and Mo powder as a conductive component on a ceramic green sheet mainly composed of Al.sub.2 O.sub.3 and then sintering this green sheet.
Recently, with integration of wiring boards, multi-layer ceramic wiring boards have attracted a great deal of attention. Multilayer ceramic wiring boards have been made by the following steps (a)-(g). That is, (a) mixing a mixture of Al.sub.2 O.sub.3 powder and a sintering assistant which is molten at 1300.degree. C.-1500.degree. C., e.g., powder of at least one compound selected from SiO.sub.2 -MgO-Al.sub.2 O.sub.3 low melting compounds, kaolinite and dolomite with an organic binder, e.g., butyral, polyvinyl alcohol, etc. and an organic solvent, e.g., trichloroethylene, alcohols, etc. to obtain a slurry material, (b) casting this slurry material by doctor blade to form a ceramic green sheet of the disired thickness, (c) making through-holes (of 200 .mu.m or more in diameter according to the present techniques) at desired positions of this green sheet by drills and the like, (d) screen printing on this green sheet a wiring pattern (of 10-20 .mu.m in thickness according to the present techniques) with a conductor paste composed of a conductive component selected from W powder and Mo powder, an organic binder, e.g., ethylcellulose, nitrocellulose, and an organic solvent, e.g., .alpha.-terpineol, n-butyl carbitol and simultaneously filling said conductor paste in the through-holes, (e) stacking the desired number of said green sheets with the through-holes connected in registration between the sheets and pressing them at 80.degree. C.-150.degree. C. at which the organic binder in said green sheets is softened to make them into a laminate, (f) sintering this laminate at 1550.degree. C.-1600.degree. C. in a reducing atmosphere and (g) plating the conductor paths exposed on the surface with Ni or Ni-Au to make connection of components easy and simultaneously to improve corrosion resistance.
However, when multi-layer ceramic wiring boards are produced in the manner as mentioned above, the green sheet part is sintered with great contraction at temperatures from melting temperature of the sintering assistant to sintering temperature of Al.sub.2 O.sub.3 and the wiring pattern part into which the sintering assistant pentrates from the surrounding green sheet part is also sintered with a great contraction like the green sheet part. On the other hand, since the sintering assistant does not penetrate into inner portion of the conductor paste part filled in the through-holes from the surrounding green sheet part, the conductor paste part is not sufficiently sintered as compared with the green sheet part and hence does not become dense and furthermore contraction of the paste part is small and hence a large strain remains in the vicinity of the through-holes after sintering, which causes cracks and warpage of the wiring board. Furthermore, when the conductor paths on the surface of multilayer ceramic wiring boards are plated, plating solution penetrates into the sintered conductor in the through-holes and this plating solution corrodes the through-holes to result in high resistance and finally disconnection. [See "Electron Components Conf. 30th ('80)" 283-285 and "Proc 23rd ECC ('73)," 204-211].